1. The feather taken from the body of a bird used for tying flies
2. The process of twisting the feather around the hook shank

In Essex the word is sometimes pronounced "ackle" where the "h" is silent.

The neck and cape feathers of the common hen (or more exotic species) these feathers are used to produce a leg effect on many flies. The cock feathers are used for Dry flies (being shorter and stiffer) and the hen feathers are used for Wet flies (being softer and longer.
See also hackle points